The present invention relates in general terms to a telecommunications device for sending and receiving waves having wavelength λ, typically used in a spectral range including radio frequencies and microwave frequencies. More particularly, the invention relates to a multiband antenna.
In known manner, the size and the weight of wireless communications systems, such as multimode terminals (terrestrial, satellite) or pocket telephone terminals, are continuously being reduced by ever greater integration of electronic circuits. For example, for pocket telephone terminals, it is desired to improve user mobility. For this purpose, efforts have been made to obtain an antenna of transverse dimensions and of height that are small, the antenna still remaining the bulkiest part of such systems. Furthermore, the appearance of the antenna should not be unsightly for the user, so hiding it completely has also become a priority.
Miniaturizing an antenna influences firstly the resonant frequency of the overall structure, which frequency is offset towards higher frequencies. In addition, miniaturization directly influences the radio properties of the structure, constituted mainly by matching, appearance of the radiation pattern, and passband. Reducing the size of an antenna generally leads to matching that is difficult, a reduction in efficiency, degradation of the radiation pattern associated with high sensitivity to the surroundings, and above all a major decrease in the width of the passband because of a Q factor that is high.
A compromise is generally made between antenna performance (good matching, controlled omnidirectional radiation, large passband) and overall size, complexity of the structure, and cost.
The objective of miniaturization generally leads to superposing two plane type radiating elements, creating a resonant frequency that depends on their dimensions. The planes are connected to a ground plane of dimensions that are larger, but nevertheless as small as possible, thus making it possible in particular to limit the sensitivity of the antenna to the surroundings.
U.S. Pat. No. 5,986,606 discloses a miniature antenna. In the second embodiment described therein, the height of the antenna is about 4.5 millimeters (mm). It comprises a ground plane having superposed thereon in parallel a “lower” rectangular radiating plane, and above that a “upper” rectangular radiating plane of the same dimensions. It has an operating frequency f1. The planes are interconnected by a substantially square short circuit plane at the bottom sides of their widths, and placed beside one of the long sides of said planes. A thin sheet of air fills the space between the bottom plane and the ground plane. Another thin sheet, this time of dielectric material having relative permittivity ∈r greater than 1 fills the space between the lower plane and the upper plane. The upper plane is also connected by a short circuit parallel to the short circuit plane. These short circuits lengthen the electrical length so as to lower the frequency f1. A primary signal source feeds the lower plane. Both planes are thus of the active type. In addition, each of the radiating planes possesses a broad slot made in the width direction, of length shorter than the width. These slots are of the same dimensions, parallel, and made in the same position in each plane which thus becomes C-shaped. Like the short circuit elements, they lengthen the electrical length and thus lower the frequency f1. The “double C” antenna operates at a frequency around 1.5 gigahertz (GHz) with a narrow passband of 0.5% for a standing wave ratio (SWR) less than or equal to 2.
Such an antenna can operate in a “high” frequency range, e.g. corresponding to the digital cellular system (DCS) standard of 1710 megahertz (MHz) to 1880 MHz, or the personal communication system (PCS) standard operating in the range 1850 MHz to 1990 MHz, without, a fortiori, being of two-band nature. Thus, that antenna cannot operate simultaneously in the “high” band and in a “low” frequency band corresponding for example to the global system for mobile communications (GSM) standard (890 MHz to 960 MHz) or to the advanced mobile phone system (AMPS) standard (824 MHz-896 MHz).